An exhaust gas discharged from a diesel engine includes particulate matter (hereinafter, referred to as “PM”). The exhaust gas regulation for preventing the air pollution caused by the PM has become stricter each year. As a measure for meeting the regulation, a diesel vehicle is provided with a filter for filtering the exhaust gas in order to reduce an amount of pollutants to be discharged into the atmosphere. This filter is called DPF (Diesel Particulate Filter).
The PM adheres to the DPF and the amount of adhered PM increases in proportion to the travel distance of the diesel vehicle. When the amount of adhered PM increases, the discharge amount of exhaust gas decreases and the original performance of the diesel engine is not exhibited, which leads to deterioration of fuel economy. In addition, when the PM adheres to the DPF to the extent that the exhaust gas cannot be discharged completely, the DPF itself is damaged, which leads to not only an outflow of the unfiltered exhaust gas into the atmosphere but also a damage of the diesel engine itself.
In order to avoid such a situation, many diesel vehicles currently on the market have pressure sensors installed in the inlet side and the outlet side of the exhaust gas of the DPF, and the exhaust state of the DPF is monitored by measuring the pressure difference therebetween (differential pressure). However, in this monitoring method that utilizes the differential pressure, it has been known that the measurement may not be correctly performed since the monitoring is performed by mechanical mechanism which is susceptible to the influence of vibration of the engine or running.
In addition, when the PM (most of which contains carbon) adhered to the DPF is burned away using, for example, diesel oil, the amount of adhered PM (hereinafter, referred to as “PM accumulation amount”) is unknown in the above method. Thus, it is difficult to estimate the amount of fuel required to remove the PM.
Thus, there has been proposed a method in which a radio wave of 0.4 GHz to 2.5 GHz is propagated into a metal housing equipped with the DPF and the PM accumulation amount is estimated by measuring the frequency characteristics of the S parameter at a resonance frequency (eigenvalue) determined by the size or the shape of the metal housing.
Related techniques are disclosed in, for example, International Publication Pamphlet No. WO 2010/074812, U.S. Pat. No. 9,399,185, and Japanese Laid-open Patent Publication No. 2011-128002.